The present invention relates generally to methods and apparatus for improving the performance of alternating current (AC) machines and, more specifically, to methods and apparatus for improving the pull-out torque, power factor, and efficiency of synchronous reluctance machines through the controlled injection of harmonic excitation into the stator winding of the synchronous machine.
Conventional polyphase AC machines, both induction and synchronous, are typically operated by a single frequency source. These machines have stator windings to which the single frequency sine waves are applied. The performance characteristics of conventional polyphase AC machines include a sinusoidal air-gap flux wave of constant amplitude rotating around the stator winding within the air-gap at synchronous speed. In this theoretical, ideal polyphase AC machine, the constant amplitude flux wave produces a constant electromagnetic torque. The torque of the machine is monotonically dependent upon this constant amplitude flux wave. This ideal situation is approximated in large scale machines.
In conventional AC machines, the magnetic flux per pole of the machine is proportional to the area of 1/2 sine wave of the air-gap flux wave of the machine. Typically, a conventional AC machine is designed to operate with at least one of the magnetic members, the iron core or teeth, of the machine at flux saturation. Thus, the saturation flux densities of the iron, or other magnetic members, of the stator and rotor determine the maximum amplitude of the air-gap flux wave. Therefore, the amplitude of the fundamental flux sine wave determines the maximum power output of a conventional machine. This is true even though maximum use is not made of all of the flux capability of the magnetic members. It should be appreciated that operation of the machine in this saturated region results in undesirable core losses and magnetization currents.
Moreover, conventional AC machines typically produce ordinarily undesirable space harmonics in their air-gap. These naturally-arising space harmonics occur as a function of the particular machine design when excited by a fundamental frequency. Factors such as slots in the machines and core saturation contribute to the generation of these undesirable space harmonics; however, they are primarily produced by the interaction between the stator flux wave and the nonuniformly distributed air-gap permeance. These space harmonic flux waves are undesirable because they typically rotate in the air-gap at speeds other than that at which the fundamental flux wave rotates. Additionally, the space harmonic flux waves rotates in both forward and backward direction, as well as at different speeds, relative to the fundamental flux wave.
For example, a naturally-arising fifth space harmonic flux wave rotates in a reverse direction relative to the fundamental flux wave at 1/5 the speed of the fundamental flux wave. Similarly, a naturally-arising seventh space harmonic flux wave travels in the same direction as the fundamental flux wave, but at 1/7 the speed.
These space harmonic flux waves interact with the squirrel cage winding in an induction motor, or with the damper winding in a synchronous motor, to produce a braking torque that reduces the useful output of the machine. Additionally, these naturally-generated space harmonic flux waves interact with each other and with the fundamental flux wave to cause pulsations in the torque of the machine, as well as undesirable mechanical vibrations.
Accordingly, the present invention provides a new method and apparatus for constructing and operating a polyphase AC machine whereby harmonic excitation is applied to the stator windings to produce a harmonic flux wave that is phase controlled to interact with the naturally occurring harmonic flux wave and enhance the pull-out torque, power factor, and efficiency of the machine.